An electrostaic precipitator is an apparatus used to remove fly ash particles from flue gas in order to reduce atmospheric pollution. The precipitator utilizes a corona discharge to electrically charge the fly ash particles, which are then attracted to a grounded collecting plate. The performance of the precipitator is in part dependent upon the electrical resistivity of the fly ash particles; the performance is most efficient when the resistivity is in the approximate range of 10.sup.9 to 10.sup. ohm cm. If the resistivity is too high, current flow through the precipitated ash layer on the plate establishes corona in the ash layer, which is detrimental to the precipitation process. Precipitation is impeded by this additional corona (termed "back corona") because the fly ash particles are subjected to bipolar charging at a diminished electric field in the interelectrode space. On the other hand, if the resistivity is too low, the particles collected on the plate are difficult to retain on the plate. As a result, they tend to be reentrained in the flue gas. When a favorable resistivity is achieved, the problems of back corona and re-entrainment are alleviated and there is a resultant increase in the precipitation efficiency.
It is known that the interaction of fly ash with sulfur trioxide enhances the performance of a cold side electrostatic precipitator by lowering the resistivity of the ash particles. Various methods are known for providing sulfur trioxide to flue gas. For example, sulfur trioxide has been provided to the flue gas upstream of the precipitator by coverting sulfur dioxide contained in the flue gas to sulfur trioxide (by means of catalytic oxidation with oxygen, after removal of fly ash, or direct oxidation with ozone), by converting sulfur dioxide from an external source to sulfur trioxide by means of catalytic oxidation with oxygen and introducing the sulfur trioxide into the flue gas, by the addition of the decomposition products of sulfuric acid (water vapor and sulfur trioxide), and by the addition of the vapor of sulfuric acid.
One disadvantage of the known methods is that the system for catalytic oxidation of sulfur dioxide to sulfur trioxide has a much higher capital cost than the system required for the present invention. The method which employs sulfuric acid as a conditioning agent is undesirable because sulfuric acid is highly corrosive.
Still other processes for providing sulfur trioxide to flue gas to enhance electrostatic precipitation involve the addition of sulfur trioxide to the flue gas in combination with ammonia and water, specifically as the decomposition products of the compound ammonium sulfate or ammonium bisulfate. U.S. Pat. No. 3,665,676 describes the addition of a finely divided powder or an aqueous solution of ammonium sulfate or ammonium bisulfate to flue gas upstream from a precipitator and downstream from an air preheater at a temperature in the 240.degree. F. to 800.degree. F. range. A disadvantage of this method is that at this point of injection the upper limit of the temperature range will rarely exceed 400.degree. F., and as a result, the thermal decomposition of the added chemical to produce sulfur trioxide will be minimal. The added ammonium sulfate may not be decomposed much further than the products ammonia and ammonium bisulfate and the added ammonium bisulfate may not be appreciably decomposed at all. With sulfur trioxide remaining in a chemically combined form (ammonium sulfate or ammonium bisulfate), it is less satisfactory than that of the equivalent amount introduced as gaseous decomposition products because either it is required in a greater amount for a given resistivity change or it imparts an undesirable sticky quality to the ash.
U.S. Pat. Nos. 4,042,348 and 4,043,768 describe methods involving the addition of an aqueous solution of ammonium bisulfate (4,042,348) or ammonium sulfate (4,043,768) to the flue gas at a point upstream from the air preheater, where the temperature is between 1094.degree. F. (590.degree. C.) and 1652.degree. F. (900.degree. C.) but preferably not above 1382.degree. F. (750.degree. C.). While U.S. Pat. Nos. 4,042,348 and 4,043,768 disclose that the temperature range of 590.degree. C. to 900.degree. C. is sufficient to bring about the volatilization of the chemicals, which may avoid air preheater pluggage, it is evident that to the degree that gaseous ammonia, water, and sulfur trioxide are produced in the volatilization process, recombination to ammonium bisulfate or ammonium sulfate may still ensue and result in air preheater pluggage.
The invention described and disclosed in U.S. Pat. No. 4,533,364, entitled Method For Flue Gas Conditioning With The Decomposition Products Of Ammonium Sulfate Or Ammonium Bisulfate, which issued Aug. 6, 1985 to Electric Power Research Institute, the assignee of the present application, sought to overcome the problems of the prior art methods by decomposing an aqueous solution of ammonium sulfate or ammonium bisulfate in a slipstream of hot flue gas or hot combustion air having a temperature in the range of about 600.degree. F. to 1000.degree. F. in a chamber external to the main flue gas duct, and then injecting the decomposition products into the main flue gas duct at a point between the air preheater and the electrostatic precipitator (ESP). The SO.sub.3 thus produced gives markedly effective improvement in ESP performance at 300.degree. F. (the typical temperature of the flue gas at the inlet to the ESP). The aqueous solution of ammonium sulfate or bisulfate is sprayed into the slipstream of hot flue gas or hot combustion air at a rate sufficient to produce a SO.sub.3 concentration in the range of from about 250 to 2500 ppm by volume.
Typically, the SO.sub.3 concentration in the slipstream is about 1000 ppm, but it would only be about 10 to 20 ppm in the main gas stream if its concentration were based on dilution alone. Data collected at power plants using this method of flue gas conditioning reveals that much less than 1 ppm of SO.sub.3 remains in the gas phase at 300.degree. F. at the inlet to the ESP. The presumption is that SO.sub.3 disappears from the gas stream by two competitive processes: (a) absorption by the fly ash and (b) recombination with NH.sub.3 and H.sub.2 O as ammonium sulfate or bisulfate particulate. The observed improvement in ESP performance seemingly has to signify that the relative rates of the two processes are such that (b) does not nullify (a). Yet if (b) could not occur at all, the benefit of (a) would be greater. The process of the present invention has solved the problem of SO.sub.3 recombination with NH.sub.3 and H.sub.2 O, thereby greatly improving the process disclosed in U.S. Pat. No. 4,533,364.
Like the process disclosed in U.S. Pat. No. 4,533,364, the present invention solves the problems of air preheater pluggage caused by injecting aqueous ammonium sulfate or bisulfate into the main flue gas stream upstream from the air preheater, and solves the problems of inadequate thermal decomposition of the ammonium sulfate or ammonium bisulfate when the aqueous solution is injected into the main flue gas downstream of the air preheater where the temperature is typically too low for complete thermal decomposition of the ammonium sulfate or bisulfate. In addition, the process of the present invention greatly improves the process disclosed in U.S. Pat. No. 4,533,364 by solving the problem of recombination of NH.sub.3 and SO.sub.3 at the lower temperatures in the main flue gas stream where the slipstream of flue gas or hot air containing the decomposition products is re-introduced downstream of the air preheater and directly upstream of the ESP.
The present invention solves the problem of NH.sub.3 and SO.sub.3 recombination by adding a catalytic element in the lower part of the decomposition chamber to destroy the NH.sub.3 gas produced by the decomposition of ammonium sulfate or bisulfate. The removal of the NH.sub.3 produces the following possible benefits: (a) elimination of the competition by NH.sub.3 for the SO.sub.3 at a reduced temperature to allow substantially all of the SO.sub.3 to react with fly ash; (b) elimination of a significant amount of NH.sub.3 that would otherwise be present in the flue gas; and (c) elimination of NH.sub.3 from the fly ash, making the ash more generally acceptable for marketing as a component of cement.